Short laser pulses are required in numerous applications. Fiber lasers that are commercially available today at comparatively low cost are typically used for the generation of short lasers pulses at a variety of different wavelengths.
Fiber lasers are lasers using optical fibers as gain media. In most cases, the gain medium is a fiber doped with rare earth ions such as erbium (Er3+), neodymium (Nd3+), ytterbium (Yb3+), thulium (Tm3+), or praseodymium (Pr3+), and one or several fiber-coupled laser diodes are used for pumping. The wavelength of the generated laser radiation depends on the dopant of the gain medium. Further wavelengths can be reached by non-linear frequency conversion, such as, e.g., second harmonic generation.
However, in the range of wavelengths accessible with mature, available rare earth doped fiber lasers and their second harmonic frequencies there is a gap extending between 800 nm and 1000 nm and between 1100 nm and 1500 nm. On the other hand, it is particularly these wavelength ranges in which there is an increasing demand for laser sources, as e.g. in the field of Biophotonics.
A well-known means to generate laser radiation at ‘uncommon’ frequencies is non-linear four-wave mixing (FWM) in step index fibers [2] or in photonic crystal fibers [1]. In these approaches, optical parametric amplifiers (OPAs) are usually pumped by a picosecond laser with a narrow bandwidth. However, a downside of this technique is that picosecond fiber lasers are difficult to set up, mainly due to the absence of compact pulse compression schemes.
On the other hand, ultrafast laser platforms are readily available on the market which are adapted for generating femtosecond pulses. These are, e.g., rare earth doped fiber lasers emitting at 1.05 μm or 1.55 μm (with pulse durations typically in the range from 90 to 150 fs). However, it is very difficult to obtain parametric gain starting from femtosecond pulses, because supercontinuum generation dominates the non-linear frequency generation. FIG. 1a shows the evolution of the FWM process in the non-linear medium using femtosecond pump pulses (pulse duration of 100 fs) at a wavelength of 1560 nm. The non-linear medium used for parametric conversion is a highly non-linear fiber (HNLF) of known type. The parametric process is seeded by chirped femtosecond pulses at a wavelength of 1960 nm (idler signal). As it can be seen from the diagram of FIG. 1a, the generated signal radiation at 1280 nm is strongly structured from the beginning because the generation of a supercontinuum is dominating the process. It is not possible to compress the spectrum of the radiation at 1280 nm to short pulse durations and therefore it is not usable for most applications.